BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 16. GLYCOGEN AND DISACCHARIDE METABOLISM

Summary

Glycogen, a readily mobilized form of fuel reserve, is a branched polymer composed of glucose residues. Most of the glucose components in glycogen are linked by α-1,4-glycosidic bonds. Approximately every tenth residue forms a branch point via an α-1,6-glycosidic bond. Glycogen is present in large amounts in Muscles and the Liver, where it is stored in the Cytoplasm as hydrated granules. Most glycogen molecules are degraded to glucose-1-phosphate through the action of phosphorylase. The glycosidic bond between C-1 of the terminal residue and C-4 of the adjacent residue is cleaved by orthophosphate to yield glucose-1-phosphate, which can be reversibly converted into glucose-6-phosphate. Branch points are broken down by the concerted action of two Other Enzymes: transferase and α-1,6-glucosidase. The latter enzyme (also known as the debranching enzyme) catalyzes the Hydrolysis of α-1,6-bonds, resulting in The formation of free glucose. Glycogen synthesis proceeds via a different pathway. UDP-glucose, an activated intermediate in glycogen synthesis, is formed from glucose-1-phosphate and UTP. Glycogen synthase catalyzes The transfer of glucose from UDP-glucose to the C-4 hydroxyl group of the terminal residue in the growing glycogen molecule. The branching enzyme converts some of the α-1,4-bonds into α-1,6-bonds.

The Synthesis and degradation of glycogen are under the coordinated control of an amplifying cascade of reactions. Glycogen synthase is inactive when phosphorylase is active, and vice versa. Epinephrine and Glucagon stimulate glycogen breakdown and inhibit its synthesis by increasing the intracellular level of cyclic AMP, which activates protein kinase. This is followed by the phosphorylation of glycogen synthase, leading to its inactivation. Phosphorylase is also phosphorylated, but in this case, the enzyme is converted into a more active form. The phosphoryl groups of both Enzymes can be removed by the action of a single phosphatase. Glycogen synthase and Glycogen phosphorylase are also regulated by noncovalent allosteric interactions. Phosphorylase indeed serves as a key component of the liver Cell system that is sensitive to glucose levels.

The dietary reserve in plants is starch, which is similar to glycogen, differing from it only in a lower degree of branching. Starch is hydrolyzed to maltose, which consists of two glucose residues linked by an α-1,4-bond. Maltose, in turn, is hydrolyzed to glucose. Other common Disaccharides include sucrose, which is hydrolyzed into glucose and fructose, and lactose, which is hydrolyzed into galactose and glucose. Galactose and glucose linked to UDP can undergo interconversion. UDP-galactose is an activated intermediate in lactose synthesis.